1. Field of the Invention
The present invention relates generally to a brake device for a power tool.
2. The Prior Arts
Many kinds of power tools are already well known, such as portable lawn mowers. In these power tools, an engine drives an output shaft mounted with a tool, such as circular saw and chain saw. When the engine stops, although the engine does not provide any power to the output shaft, the output shaft still rotates awhile due to its inertia. At this time, some undesired things might happen. For example, the tool might further cut the machined work piece and thus influence the machining accuracy thereof, even might injure the operator.
Therefore, in order to reduce the risk of power tools, many kinds of brake device for power tools are proposed, such that the output shaft can be immediately stopped once the engine stops. Taiwan Patent Publication No. 221,936 disclosed a safety brake device for a sawing machine, in which an eccentric device and a brake plate are mounted on a spindle in the casing of the motor. An eccentric plate of the eccentric device drives the brake plate to contact with a bearing of the output shaft of the motor, thereby to achieve the purpose of braking. When the power of the motor is off, the rotational speed quickly slows down and the eccentric force reduces, such that the eccentric plate drives the brake plate to contact with the bearing to cause a friction for braking. Thus, the spindle of the motor can be stopped in a short time.
U.S. Pat. No. 4,793,064 disclosed a brake device for a chain saw, which comprises many components, such as a brake handle, a pivot member, and a brake band. The operator needs to press the brake handle to force the brake band against the output shaft, so as to make the output shaft stop due to the friction between the brake band and the output shaft.
In these brake devices for power tools mentioned above, although the output shaft can be quickly stopped once the engine stops, they still have some problems, such as components numerous, structure complicated, weight and manufacturing cost increased. Further, in U.S. Pat. No. 4,793,064, in order to quickly and completely stop the output shaft after the engine stops, the operator still needs to press the brake handle to stop it that is inconvenient in operating. Thus, it is necessary to provide a brake device for a power tool which is easy to operate, manufacture, and has a simple structure, fewer components, and reduced weight and cost.